Currently, there is great interest in the use of alternative renewable biofuels, such as biodiesel, that can be produced from biooil feedstocks that are in turn produced by oleaginous microorganisms such as microalgae, bacillus, fungi, and yeast. Oleaginous microorganisms are microbial with lipid content typically in excess of 20%. A renewable liquid fuel energy source could play a significant role in reducing our national dependence on foreign oil imports. Reported in the literature is that oleaginous yeasts and microalgae can grow and accumulate significant amounts of lipids (see A. Banerjee, R. Sharma, Y. Chisti and U. C. Banerjee, “Botryococcus Braunii: A renewable source of hydrocarbons and other chemicals” Critical Reviews in Biotechnology, 22 (3), 245-279, 2002; Y. Chisti, “Biodiesel from microalgae beats bioethanol” Trends in Biotechnology, 26 (3), 126-131, 2007; P. Metzger and C. Largeau, “Botryococcus braunii: a rich source for hydrocarbons and related ether lipids” Appl. Microbiol. Biotechnol, 66, 486-496, 2005; X. Meng, J. Yang, X. Xu, L. Zhang, Q. Nie, M. Xian “Biodiesel production from oleaginous microorganisms” Renewable Energy, 34, 1-5, 2009, the contents of each of the aforementioned papers being incorporated by reference). The oil content and composition are a function of the type of microorganisms used and the conditions in which the culturing took place. As an example, microalgae are sunlight driven cell factories that convert carbon dioxide to potential biofuels. Microalgae grow at a very fast pace, doubling their biomass within a 24 hour time period and are rich in oil. The lipid content of microalgae can be as high as 70%. In particular, microalgae are reported to be excellent candidates for biodiesel production because of their higher biomass production, higher photosynthetic efficiency, and faster growth compared to most other energy crops.
Most of the work reported in the literature on the development of microbial oil production has focused on the identification of better strains of oleaginous microorganisms, on genetic and metabolic engineering of strains, on the development of the optimal environmental conditions for microorganism growth, and on the development of the optimal energy sources to fuel the growth of the microorganisms.
Similar to the biofuel studies there have been studies on chemical and nutraceutical production in microalgae (see J. N. Rosenberg, G. A. Oyler, L. Wilkinson and M. J. Betenbaugh, “A green light for engineered algae: redirection metabolism to fuel a biotechnology revolution”, Current Opinion in Biotechnology, 19, 430-436, 2008, the contents of which are hereby incorporated by reference). However there has been little effort spent on the harvesting of microorganisms, particularly from large-scale (100 liter to 2 million liter) volume cultures. Therefore, significant challenges remain in the energy efficient and economical harvesting of microorganisms from their host medium, as well as steps to collect the microbial oils. In particular, harvesting of the microorganisms by the concentration and separation of the microorganisms from their host medium, typically water.
Algae use in bioreactors or large ponds is increasingly being employed for biofuels and nutraceuticals. Metzger, and Largeau (2005) and Banerjee (2002) describe the use of Botryococcus braunii as a source of hydrocarbons and similar lipids, such as C27diane, C30 botryococcene, squalene, tetramethylsqualene and trs,trs-lycopadine, among others including ether lipids, epoxides and sterols. Weldy and Huesemann (C. S. Weldy and M. Huesemann, “Lipid production by Dunaliella salina in batch culture: effects of nitrogen limitation and light intensity” U.S. Department of Energy Journal of Undergraduate Research, Vol. VII, 115-122, 2007) and Hejazi and Wijffels (M. A. Hejazi and R. H. Wijffels, “Effect of light intensity on beta-carotene production and extraction by Dunaliella salina in two-phase bioreactors” Biomolecular Engineering, 20, 171-175, 2003) describe the use of Dunaliella salina, for lipid production and beta-carotene production. Other researchers, including Chisti (2007), Meng et al. (2009) and Hu (Q. Hu, M. Sommerfeld, E. Jarvis, M. Ghirardi, M. Posewitz, M. Seibert, and A. Darzins, “Microalgae triacylglycerols as feedstocks for biofuel production: perspectives and advances” The Plant Journal, 54, 621-639, 2008) discuss the use of microalgae for biodeisel and triacylglycerols production. Lastly, Rosenberg et al (2008) describe a whole list of nutraceutical, pharmaceuticals, and high-value chemicals produced from microalgae. In all these applications, there is a need for improved algae concentrating, or as is known, dewatering. Conventional techniques involve batch centrifuging at high-cost and high energy.